Dewatering of sewage sludge

ABSTRACT

The present invention relates to a sludge treatment plant in which sludge is flocculated and then dewatered to form a cake and a reject liquor is modified by recycling 3 to 40% of the reject liquor into the sludge immediately before the dewatering process.

This application claims priority to provisional application No.60/044,437, filed Apr. 30, 1997.

FIELD OF THE INVENTION

This invention relates to processes and apparatus for dewatering sewagesludge so as to produce a cake and a reject liquor which can be asupernatant, filtrate or centrate.

BACKGROUND TO THE INVENTION

A typical sewage treatment plant takes in raw sewage and produces solidsand clarified water. Typically the raw sewage is treated in a primarysedimentation stage to form a primary sludge and supernatant, thesupernatant is subjected to biological treatment and then a secondarysedimentation stage to form a secondary sludge and clarified liquor,which is often subjected to further treatment before discharge.

The sludges are usually combined to form a mixed sewage sludge which isthen dewatered to form a cake and a reject liquor. The reject liquor isusually recycled to the head of the plant and the start of the process,i.e., fed back to the primary sedimentation stage or a preceding stagein the plant. Any water which is required in the plant, for instance fordissolving polymeric flocculant, is usually either potable water (fromthe local drinking water supply) or is clarified water from thesecondary sedimentation stage, optionally after any subsequent treatmentprocedures.

It is standard practice to dewater the sludge by mixing a dose ofpolymeric flocculant into that sludge at a dosing point, and thensubstantially immediately subjecting the sludge to the dewateringprocess and thereby forming a cake (typically having a solids content of15 to 35%) and a reject liquor. The dewatering process may becentrifugation or may be by processes such as filter pressing or beltpressing.

The process is generally conducted so as to obtain the highest possiblecake solids, preferably accompanied by the highest possible rejectliquor clarity. It is well established that, for any particularperformance parameter (such as cake solids) there is an optimum dosageof flocculant and that increasing the dosage above this optimum does notresult in an improvement but instead tends to result in overdosing andin deterioration of performance. Accordingly, it can be assumed thatpolymer applied at dosages of up to the optimum is substantially alladsorbed on to the sludge so as to participate in bridging flocculation,whereas overdosing of polymer is liable to introduce additional polymerinto the sludge with the result that excess polymer may remain in thereject liquor and the presence of this excess can interfere with thebridging flocculation performance.

There is a maximum solids loading rate (kg dry matter per hour) and amaximum hydraulic loading rate (liters sludge per hour) which can behandled in any particular dewatering apparatus. It has conventionallybeen regarded to be desirable normally to operate as close as reasonablypracticable to the maximum solids loading rate, and in order that thiscan be achieved without exceeding the hydraulic loading rate it isnecessary that the sludge which is dewatered should have a solidscontent within an appropriate range. If the sludge would normally have asolids content below this, it is conventional to subject it to athickening stage (often promoted by the use of polymeric flocculant)before dosing the polymeric flocculant into the resultant sewage sludgesubstantially immediately prior to the final dewatering intended to formthe cake and the reject liquor.

When setting up a dewatering process, it is therefore generallynecessary to select the optimum dose and the optimum solids loading rateand hydraulic loading rate.

Thus the optimum combination of solids and hydraulic loading rates isselected to give the most efficient utilisation of the dewateringapparatus having regard to the sludge that is being dewatered.Thereafter, in most cases, it is generally undesirable to dilute thesludge since that increases the hydraulic loading rate withoutincreasing dewatering efficiency.

In other processes, the sludge which is to be dewatered may have asolids content which is too high for optimum results, for instancebecause the viscosity of the sludge is such that it is difficult toachieve efficient mixing of the flocculant into the sludge and/or toachieve efficient distribution of the flocculated sludge in thedewatering apparatus. Under these circumstances, it is necessary todilute the sludge prior to dosing the polymeric flocculant into it. Thewater which is used for dilution is usually the same water as is usedfor other purposes in the overall plant, namely either water from thedrinking water supply or clarified water from the plant.

Various proposals have been made in the literature for recycling variousstreams to various points in a sewage treatment plant (additional to thenormal recycling of the final reject liquor to the head of the plant, asdiscussed above).

For instance, in U.S. Pat. Nos. 5,075,012 and 5,248,416 a stream ofunfiltered liquor which is approaching the dewatering apparatus andwhich contains flocculated solids is recycled to an earlier point in thefeed line so as to promote flocculation of the feed.

When, as mentioned above, it is necessary to thicken the sludge bysedimentation or filtration prior to the final dewatering which forms acake, the filtrate (or supernatant) from the thickener is sometimes notas clear as is desired. Instead of recycling all of this thickenerfiltrate or supernatant to the head of the plant, it is known to reusethe thickener supernatant or filtrate for diluting the polymericflocculant which is added either to promote the thickening or to promotethe subsequent dewatering.

In JP-A-57150480 it is proposed to hold effluent in a store tank andsubject it to sedimentation in a sedimentation tank into which polymericflocculant is added. Sludge is taken from the base of this tank and someof the supernatant from the tank is recycled to the store tank so as todilute the liquor which is being passed to the sedimentation tank. It isalleged that the amount of flocculant which is needed in thesedimentation tank is reduced because of the increased dilution of thesolids in that tank, but a disadvantage of this process is that thehydraulic load in the tank is increased.

In JP-A-58146498 sewage sludge, after aeration, is subjected tosedimentation in a sludge concentration tank in which sludgeprecipitates under gravity. The precipitated sludge is taken to acentrifugal thickener and the supernatant from the centrifugal thickeneris combined with the supernatant from the sedimentation tank. The sludgefrom the centrifugal thickener is dosed with polymeric flocculant and isthen subjected to dewatering to provide a cake and a filtrate. It seemsthat the amount of flocculant which is added is more than would normallybe regarded as optimum as it is alleged that, due to the high dosage,large amounts of flocculant are lost with the filtrate andeutrophication is a significant problem in the resultant effluent.

Filtrate from the dewatering apparatus is recycled to the sludgeentering the centrifugal thickener and/or to the sludge entering thesedimentation concentration tank and this is alleged to improve thesedimentation and/or thickening. However it should be noted that theonly point at which flocculant is added is immediately prior to thedewatering apparatus and that the sedimentation concentration and thecentrifugal thickening are both conducted without the deliberateaddition of flocculant. Since these processes were being conductedwithout flocculant and since the filtrate is said to contain wastedflocculant, it is not surprising that adding some of this filtrate tothe sludge which is to be subjected to sedimentation and/or centrifugalthickening in the absence of deliberately added flocculant would lead toan improvement in the sedimentation or thickening.

We are concerned with something entirely different, namely achieving aperformance in a dewatering process which produces a cake and a rejectliquor which is better than a performance which, prior to the invention,would have been regarded as already being optimum in that particularprocess. Thus, we are concerned inter alia with modifying a processwhich is already thought to be operating under optimum conditions(having regard to polymer dose, cake solids and reject liquor clarity)and obtaining improved cake solids and/or reject liquor clarity atconstant polymer dose or obtaining constant cake solids and/or rejectliquor clarity at reduced polymer dose. Clearly, none of the proposalsdiscussed above gives any suggestion as to how to achieve this.

SUMMARY OF THE INVENTION

According to the invention, a sludge treatment process comprises

providing a sewage sludge,

mixing polymeric flocculant into the sludge at a dosing point, and then

substantially immediately subjecting the sludge to a dewatering processand thereby forming a cake and a reject liquor, and in this process

3 to 40% by volume of the reject liquor is recycled into the sludge at adilution point substantially immediately before the dewatering process.

In normal operation of the process the dose of polymeric flocculant needbe no more than the optimum dose of that flocculant for that sludge inthat dewatering process without the recycling of the reject liquor.

A sludge treatment plant according to the invention comprises adewatering apparatus by which sludge can be dewatered into a cake and areject liquor, a sludge feed line for feeding sewage sludge into theapparatus, and a reject liquor line for leading reject liquor from theapparatus, and a recycling line for recycling reject liquor from thereject liquor line to the sludge feed line.

In this invention the reject liquor may be any type of reject liquorproduced by a dewatering process. In particular it can be a supernatant,a filtrate or a centrate.

DESCRIPTION OF PREFERRED EMBODIMENTS

In a simple aspect of the invention a sludge dewatering process isconducted under substantially optimum conditions for that particularsludge and dewatering plant, i.e., at or slightly below optimum polymerdosage and substantially at optimum hydraulic loading rate and solidsloading rate so as to obtain the substantially optimum combination ofdewatered sludge solids and reject liquor clarity if there is norecycling of reject liquor, and 3 to 40% of the reject liquor isrecycled to a dilution point substantially immediately before thedewatering process. As a result, the hydraulic load on the plant isincreased (which would generally be thought to be undesirable) but it isfound that the cake solids and/or clarity are improved if the dose offlocculant is maintained unchanged.

As a result, it is possible to modify the process by reducing the doseof flocculant and yet obtain cake solids and/or clarity equivalent towhat was obtained at the higher dose without the recycling of the rejectliquor. Depending upon the objective of the process, it is thus possibleeither to maximise cake solids and/or reject liquor clarity or to reducethe dose of flocculant, or to obtain a partial reduction in the dose offlocculant and a partial improvement in cake solids and/or reject liquorclarity. Thus, preferred processes for this invention are those whereelimination of the recycle would result in reduced cake solids and/orrequires additional polymer to maintain cake solids.

In many processes according to the invention, the dose of polymericflocculant used in the process of the invention is below 95% and oftenbelow 90% and sometimes as little as 85 or 80% of the optimum dose undernormal operation (i.e., without recycling of the reject liquor). Thus itis possible to achieve a saving in polymer which is easily 5% and can beas much as 15 or 20% while maintaining cake solids and/or reject liquorclarity.

It will be appreciated that the total amount of polymer which is beingdosed into the process is generally no greater than, and is often lessthan, the amount that is required to give optimum bridging flocculationand dewatering in a process without the recycle and so the unexpectedadvantages in the process cannot be due to reliance on polymer whichremains in the liquid phase and which is additional to what is requiredfor optimum dewatering in the corresponding process conducted withoutrecycle.

Further, the percentage reduction in polymer required to give constantcake solids and/or reject liquor clarity is often greater than the totalamount of polymer which could, on any basis, be speculated to be presentin the recycled reject liquor. For instance, if 10% of the reject liquoris recycled this represents a volume of liquid which would have carried10% of the initial polymer dose into the dewatering process. Most or allof this initial 10% of the polymer dose would have been utilised inbridging flocculation and so would be trapped in the cake, and so themaximum amount of polymer which could be present in this 10% recycle isfar less than 10% and possibly at or near zero, and yet it can typicallylead to a reduction of 5 to 20% in the amount of polymer which has to bedosed into the sludge in order to obtain equivalent dewatering. Insteadof reducing the amount of polymer, in some instances it can bebeneficial to use a less costly and less efficient polymeric flocculantat increased, similar or reduced dosage.

The reason why improved dewatering performance is obtained, includingbeing obtained at increased hydraulic loading rate, is unclear.

The invention also includes processes in which improved dewateringperformance is obtained at constant hydraulic loading rate. Thus, inanother simple process of the invention, an existing process whereinfeed sludge is diluted with potable or clarified water prior to thedewatering process is modified by replacing some or all of this dilutionwater with recycled reject liquor. For instance there can be 50 to 100%replacement of the dilution water with the recycled reject liquor. Theamount of recycled reject liquor (and any dilution water which is stillused) is often the same as the original amount of dilution water but canbe greater or less, so that the process then has an increased hydraulicload or a decreased hydraulic load when recycling in accordance with theinvention. These processes are useful when the sludge naturally has asolids content and viscosity higher than is suitable for normaloperation of the admixture of polymeric flocculant and the applicationof the subsequent dewatering process.

When the invention is applied to an existing process which is alreadyoperating with a polymer dose at or below optimum, the invention merelyrequires the addition of recycle of some of the reject liquor andpossible reduction in the amount of polymer, or the use of less polymeror a less efficient (and perhaps less costly) polymeric flocculant. Whena process is to be designed from the start in accordance with theinvention, it is desirable to select the amount of polymeric flocculantso that it is not more than the optimum for the process without therecycle. In practice the plant operator will probably insist upon thisanyway. The method of determining the optimum can be whatever iscustomary at that plant. It can be on the basis of actual operation ofthe plant and measurement of cake solids or clarity or a combination ofboth (depending upon the preference of the plant operator). It can bedetermined initially by a laboratory test which is known to beindicative of plant performance, for instance a CST test. Subsequenttesting on plant of potential systems chosen in this way can establishthe optimum type and dose of polymer.

In practice, the precise value of the optimum dose for that sludge inthat dewatering process does not have to be determined to great accuracybut instead can be estimated in accordance with the normal accuracyassociated with the operation of the plant. The dose used in theinvention preferably does not substantially exceed this optimum, asotherwise the economic advantages of the process are reduced or lost,and preferably the dose is 70 to 100% of the optimum, often 80 to 95% ofthe optimum.

If no other test is established as being preferred at a particularplant, for the purposes of this specification the optimum is establishedbeginning with systems chosen using minimum time as indicated from CST(capillary suction time) tests conducted over a range of doses. Finaloptimisation is carried out on the plant.

The optimum amount of recycle in any particular process will depend uponthe sludge and the other process conditions but is generally at least 5%and usually is not more than 30%, by volume of the reject liquor. Therecycling of 30% or higher does not appear to give any furtherimprovement in performance but does give increased hydraulic loadingrate, whereas the use of too little recycle does not give sufficientimprovement. 7 to 25% is often a suitable range.

The process can be operated with a fixed amount of recycle, butpreferably the amount of recycle is varied either to give a constantsolids loading rate or to give a constant hydraulic loading rate or togive some predetermined combination of solids loading rate and hydraulicloading rate. Accordingly, the amount of recycle can be varied inresponse to the rate of supply of the sewage sludge and/or to the drymatter content of the sewage sludge. By this means it is possible notonly to obtain the benefits of the invention but also the improvedperformance that comes from having a more uniform solids loading rate orhydraulic loading rate or both in the dewatering process. Preferably thedry matter content and/or the rate of flow of the sludge is monitoredin-line continuously or intermittently and the monitored values areautomatically utilised to adjust the amount of recycle, for instanceusing techniques broadly as described in PCT/GB96/00814.

A conventional sludge treatment plant has a sludge feed line whichincludes a sludge pump by which the sludge is pumped past the flocculantdosing point (which can in fact include two or more dosing points) andinto the dewatering apparatus. The recycle can lead into any pointsubstantially immediately before the dewatering process, i.e.,substantially before the inlet to the dewatering apparatus. Accordinglyit can be between the dosing point and the dewatering process (orbetween dosing points if there is more than one such point), between thesludge pump and the dosing point or before the sludge pump. If it is atthe polymer dosing point, then the recycle can be used as part or all ofthe dilution water for the polymeric flocculant in which event theprocess may be operated without any increased hydraulic loading rate andmay consist merely of the use of the recycle for polymer dilutioninstead of water from the drinking water supply or clarified water forpolymer dilution. Preferably, however, the recycle is to a point beforethe dosing point. Preferably the recycle is to the suction side of thesludge pump, either direct into the pump or into the sludge line aheadof but close to the pump.

The recycle point (and the polymer dosing point) must both besubstantially immediately before the inlet to the dewatering plant sinceit is well known that the benefits of adding treatment chemicals to amaterial which is to be dewatered are generally lost if the addition ofthe treatment chemical is conducted too long before the start of thedewatering process. Thus the point at which the recycle is added to thesludge feed and the point at which the polymer is added to the sludgefeed are generally both such that the sludge enters the dewateringapparatus within a few seconds or, at the most, a few minutes after theaddition. Typically therefore the recycle and the addition of thepolymer are both conducted at a time which is less than 10 minutes andusually less than 5 minutes and most preferably less than 2 minutesbefore the time when the sludge enters the dewatering apparatus.

The method of recycle should be conducted in such a manner that theamount of recycle can easily be controlled accurately and such that itis substantially unaffected by back pressure from the sludge line intothe recycling line. This difficulty is minimised when, as is preferred,the recycle is to the suction side of the sludge pump but if the recycleis to the sludge line between the pump and the dewatering apparatus itmay be necessary to install an arrangement which will ensure that thedesired controlled amount of recycle into the sludge can be introducedagainst the prevailing pressure.

The dewatering apparatus can be any suitable apparatus by which sewagesludge can be dewatered to form a cake and a reject liquor. Thus it canbe a dewatering centrifuge of the type which will produce a cake and acentrate, for instance a KHD Humboldt or Alfa Laval Sharples (tradenames) centrifuge. Alternatively it can be a belt press or a plate andframe press. Novel apparatus according to the invention comprises anysuch dewatering apparatus (equipped with its conventional inlet feedline and outlet reject liquor line) wherein means for recycling acontrolled proportion of reject liquor are provided leading from theoutlet reject liquor line to the inlet feed line. This means maycomprise suitable valving and flow meters and other control means fordiverting a controlled proportion of the reject liquor back into theinlet in response either to manual operation or to automated operation,for instance automated in response to measurement of the rate of flow orthe solids content, or both, of the sludge approaching the dewateringapparatus.

The sludge can be any conventional type of sewage sludge and isfrequently a blend of primary and secondary sludges. The sludge which isfed to the dewatering process usually has a solids content above 0.5 or1% by weight and usually above 2 or 3%. Generally it is not more thanabout 5 or 6% but in some instances it can be as much as 8% or even 10%,depending upon the nature of the sludge.

If the sludge, as initially supplied from the primary and/or secondarystages, does not have appropriate solids content then the solids contentmay be adjusted before approaching the dewatering process of theinvention. This adjustment may be by dilution or by thickening. Thethickening can be assisted by the use of a polymeric flocculant. Thethickening results in the formation of a filtrate or supernatant (whichis usually recycled to the head of the plant) and the thickened sludgewhich is then used in the invention. The thickening can be by, forinstance, sedimentation, gravity filtration or centrifuging and resultsin the production of a thickened sludge (in contrast to the dewateringprocess of the invention which produces a cake).

The polymeric flocculant can be a single polymer added at one or moredosage points or it can be different polymers, generally addedsequentially. It may be used in combination with an inorganic coagulant,in known manner. Usually part or all of the polymeric flocculant is awater soluble synthetic polymer usually formed from a water solubleethylenically unsaturated monomer or monomer blend. It may be anionic,non-ionic or cationic. Often the flocculant is formed from a blend ofnon-ionic monomer (such as acrylamide) and cationic monomer, such asdialkylaminoalkyl (meth)-acrylate or -acrylamide or diallyl dimethylammonium chloride). However any of the other conventional cationic oranionic or non-ionic sludge flocculants can be used in the invention.The molecular weight of synthetic polymers used as flocculant is usuallyhigh, for instance indicated by an intrinsic viscosity (suspended levelviscometer at 20° C. in 1N sodium chloride solution buffered to pH7)above 4 and often 6 to 30 dl/g, usually 6 to 15 dl/g when the polymer iscationic and 10 to 30 dl/g when it is non-ionic or anionic.

The polymer is usually provided initially to the plant as powder orreverse phase emulsion (which can be anhydrous) and is usually activatedin water before addition to the sludge, all in conventional manner. Asindicated above, some or all of the water used for the activation can bepart or all of the reject liquor recycle.

The invention also provides, in a second aspect, an improved thickeningprocess for sewage sludge comprising

providing a sewage sludge,

mixing polymeric flocculant into the sludge at a dosing point and then

substantially immediately subjecting the sludge to a thickening processand thereby forming a thickened sludge and a reject liquor, wherein

30 to 40% by volume of the reject liquor is recycled directly into thesludge at a dilution point substantially immediately before thedewatering process and before the flocculant dosing point.

In this aspect of the invention improvements are found in thickeningprocesses by using a principle similar to that of the first aspect ofthe invention for dewatering. All process features of the dewateringprocess of the invention may be applied to the thickening process of theinvention where relevant.

The invention also provides, in a third aspect, a process for improveddewatering of a cellulosic sludge comprising

providing a cellulosic sludge,

mixing polymeric flocculant into the sludge at a dosing point and then

substantially immediately subjecting the sludge to a dewatering processand thereby forming a cake and a reject liquor, wherein

3 to 40% by volume of the reject liquor is recycled into the sludge at adilution point substantially immediately before the dewatering process.

In this aspect of the invention the cellulosic sludge can be forinstance paper mill effluent sludge or de-inking plant sludge.

All process features of the dewatering process of the first aspect ofthe invention may be applied to the dewatering process of the thirdaspect of the invention where relevant.

The invention is illustrated in the accompanying drawings in which

FIG. 1 is a schematic representation of a typical sewage treatment plantshowing, in dashed lines, the modification of the plant in accordancewith the invention and

FIG. 2 is a schematic representation of a dewatering plant according tothe invention.

Referring to FIG. 1, raw sewage enters at 1 and passes through varioussteps including primary clarification 2, secondary biological processing3, secondary clarification 4, sludge thickening 5, sludge digestion 6and sludge dewatering 7. Some primary sludge from the primaryclarification is sent directly to the feed line to the sludge digestionprocess 6 whilst the remainder passes to secondary biological processing3. Secondary clarification 4 produces waste activated sludge of whichsome passes to sludge thickening 5 (for which purpose polymer is addedto the sludge line) and some is sent back into the feed line forsecondary biological processing. Reject liquor from secondaryclarification 4 is passed to tertiary filtration 8 and disinfection 9before discharge as plant effluent. The sludge dewatering processproduces a sludge cake which can be disposed of in various ways and areject liquor which is passed back to the head of the plant at 1.Similarly excess reject liquor from the sludge thickening process ispassed to the head of the plant 1. The dotted lines show the recyclingof reject liquor from sludge dewatering 7 to the sludge line enteringsludge dewatering.

The sludge from the primary and secondary sedimentation stages isblended and, as shown in FIG. 2, is pumped along a sludge feed line 20by a sludge pump 21 past a polymer dosing point 22 at which dissolvedpolymer is supplied from a line 23, from a polymer solution feed 28 andvia a polymer solution feed pump 29. The addition of the polymer intothe sludge line is conducted in conventional manner so as to obtainrapid mixing of the polymer into the sludge as it flows along the linetowards a dewatering apparatus 24. It enters this at an inlet 25. Rejectliquor is taken out of the dewatering apparatus by outlet line 26.Removal means can include drums (not shown). Cake is removed inconventional manner by cake removal means shown diagrammatically as 27.In accordance with the invention, recycle is provided between the outletline 26 and the sludge line 20, and in the illustration the recycle isto the suction side of the sludge pump 21. It is also possible torecycle to the discharge side of the pump 21 at point 33 (shown as adotted line). Recycle is also shown to the polymer line 23. Each recycleline 26 is provided with a valve 32 which enables switching off of thatrecycle, and if desired with a flowmeter 30 and calibration cylinder forcontrol purposes. In order to control the amount of recycle, suitablecontrol apparatus comprises a variable speed progressive cavity pumpcontrolled manually or automatically through the feedback signal of aflow measuring device or solids sensing device mounted on the sludgefeed line.

The following are examples of the invention.

EXAMPLE 1

A mixture of primary and secondary sludge was being pumped at 150gallons per minute along a sludge line to a centrifugal dewateringapparatus, with the addition of 25 gallons per minute of potable waterto the sludge line so as to reduce the viscosity of the sludge and withthe addition as solution of 22.6 pounds polymer per dry ton of sludge asflocculant. The polymer was a copolymer of acrylamide anddialkylaminoalkyl acrylate quaternary salt having IV in the range 6 to12. The amount of the polymer had been optimised so as to provide cakesolids which, on a general basis, were about 18 to 20% and good centrateclarity. 22 gallons per minute of the centrate were then recycled intothe sludge line as a replacement for the 25 gallons per minute potablewater and the amount of polymer was reduced to about 15 pounds, areduction of almost 30%. Cake solids dropped from a value of about 20%immediately before the change to about 18% and the centrate clarity wasexcellent. The recycle was then terminated and replaced by dilution with25 gallons per minute potable water and the amount of polymer wasincreased back to about 18 pounds, whereupon the cake solids increasedby less than 1%.

This trial shows that cake solids can be maintained at about theprevious level while reducing the polymer dosage significantly if thedilution with potable water is replaced by recycle of centrate. In thisparticular example the amount of recycle is about 15% by volume.

Interestingly, attempts to reproduce these results by laboratory testsfailed, but it is not clear whether this was due to a temporary failurein the laboratory techniques or whether it is indicative of theinvention relying, in some unexplained manner, on the dynamics and otherplant process conditions that prevail during an actual dewateringprocess on a plant. In this plant, the dewatering was by a centrifuge.

EXAMPLE 2

Dewatering was being carried out on a mixture of primary and secondarysludge having a solids content of around 5 to 6% using dewatering plantin which the dewatering apparatus was a belt press. The process wasoperating under conditions which the plant considered to be optimum,utilising no dilution water but the addition of 4.8 pounds per dry tonof a high molecular weight cationic polymeric flocculant added as asolution.

Trials were conducted on three consecutive days, and in each instancethe trial consisted of operating under optimum conditions withoutdilution water or recycle of filtrate followed by recycle of filtrate(in amounts of around 17 to 21%) at the same or decreased polymerdosage, and the cake solids was observed. In each instance the filtrateclarity was also observed and was maintained acceptable throughout. Therecycle of filtrate was made to the suction side of the sludge pump. Theresults are set out in the following table.

Polymer Sludge Feed Dosage Cake Feed Rate Solids (lb/dry Recycle Solids(gpm) (% T.S.) ton) (gpm) (% T.S.) 90.0 5.54 4.81 0 29.72 97.0 5.52 4.8117.4 32.98 97.0 5.21 3.85 17.4 28.89 88.0 4.2 5.87 0 26.18 93.0 4.2 5.8719.6 29.46 93.0 4.36 4.76 19.6 25.31 90.0 3.44 4.9 0 24.99 97.0 3.44 4.919.2 28.31

It is apparent from these results that the recycle gave increased cakesolids when the polymer dosage was unchanged or equivalent cake solidsat reduced polymer dosage. In particular increases in cake dryness, atunchanged polymer dosage, of around 11, 12 and 13% were recorded, andreductions in polymer dosage of around 7 and 19% were recorded atequivalent cake solids.

What is claimed is:
 1. A sewage sludge treatment process comprising a)pumping sewage sludge with a sludge pump through a sludge line to adewatering apparatus b) mixing polymeric flocculant into the sludge at adosing point in said sludge line and then c) substantially immediatelysubjecting the sludge to a dewatering process in said dewateringapparatus and thereby forming a cake and reject liquor, wherein 3 to 40%by volume of the reject liquor is recycled into the sludge line at asuction or discharge side of said sludge pump at a dilution pointsubstantially immediately before the dewatering process, in which theamount of polymeric flocculant dosed into the sludge is no more than theoptimum dose of that flocculant for that sludge in that process withoutthe recycling of the reject liquor.
 2. A process according to claim 1 inwhich the dewatering is by centrifugation, filter pressing or beltpressing.
 3. A process according to claim 1 in which the amount ofreject liquor which is recycled is 5 to 20% by volume of the rejectliquor.
 4. A process according to claim 1 in which the amount of rejectliquor which is recycled is controlled automatically in response to theflow rate of the sludge to the dewatering apparatus and/or to the solidscontent of the sludge.
 5. A process according to claim 1 in which thereject liquor is recycled to the suction side of the sludge pump.
 6. Aprocess according to claim 1 in which the recycle is to a position atwhich the sludge is within 10 minutes of being subjected to thedewatering process.
 7. A sewage sludge thickening process comprising a)pumping sewage sludge with a sludge pump through a sludge line to athickening apparatus, b) mixing polymeric flocculant into the sludge ata dosing point in said sludge line; and then c) substantiallyimmediately subjecting the sludge to a thickening process in saidthickening apparatus and thereby forming a thickened sludge and a rejectliquor, wherein 3 to 40% by volume of the reject liquor is recycled intothe sludge line at a suction or discharge side of said sludge pump at adilution point substantially immediately before the thickening processand before the flocculant dosing point, in which the amount of polymericflocculant dosed into the sludge is no more than the optimum dose ofthat flocculant for that sludge in that process without the recycling ofthe reject liquor.
 8. A cellulosic sludge treatment process comprisingpumping cellulosic sludge with a sludge pump through a sludge line to adewatering apparatus, mixing polymeric flocculant into the sludge at adosing point in said sludge line and then substantially immediatelysubjecting the sludge to a dewatering process in said dewateringapparatus and thereby forming a cake and reject liquor, wherein 3 to 40%by volume of the reject liquor is recycled into the sludge line at asuction or discharge side of said sludge pump at a dilution pointsubstantially immediately before the dewatering process.